A. Field of the Invention
This invention relates to electronic digital control systems for controlling solid state rectifiers in a reversible bridge configuration connected between an AC power source and a DC electric motor; and more particularly, to a method for controlling the firing angle of the controllable rectifiers regulating the transition from the forward conducting bridge to the reverse conducting bridge.
B. Background Discussion
Numerous DC electric motors are found in industrial use where variable speed, reversing, positioning or battery operation is needed. A DC motor is well suited for where precise control and position is desired. For example, a DC electric motor which powers an elevator requires a very precise control system to accurately stop the elevator and align the floors of the elevator and the building. In stopping the elevator, it is first necessary to excite the DC electric motor in the reverse direction prior to a complete stop. During the reversal it is very desirable to maintain as much control of the DC electric motor as possible and to provide a smooth continuous transition from the forward to reverse direction.
As is well known, this type of control system is well suited for electronic servo system control. The servo system control is typically designed as a closed loop system with a speed controller as the outer servo loop and a current controller as the inner servo loop. The control system is usually designed with the DC electric motor fed from a three phase bridge convertor having a first and second controllable bridge rectifier power regulators activated by the speed and current controllers.
In the past, controlling the conduction of the rectifiers have generally been accomplished by using analog control devices to perform the regulating functions required and converting the analog signal into digital values to activate the rectifiers. These analog devices invariably are proportional plus integral controllers for the speed and current loops. These controllers regulate the speed of the DC electric motor by controlling the activation of antiparallel-connected, three-phase dual silicon control rectifiers (SCR). Six SCRs form a forward bridge to drive the motor in the forward direction and six SCRs form a reverse bridge to drive the motor in the reverse direction. The point, as an angle, along the control voltage half-cycle at which an SCR activates is known as the firing angle.
In analog systems the manner of controlling the speed of a DC motor, when operating in either continuous or discontinuous current mode is well known, as is the manner of reversing the direction of the motor. It is also known that one criteria for reversing the direction of a DC electric motor is that the motor current be zero at the time of reversal. In analog systems, in order to make this reversal it is first necessary to detect when the current is zero and then to wait a specified safe period before reversing the motor.
Driven by these bridges, the DC electric motor can operate under conditions of both positive and negative values of motor voltage and motor current. The foregoing conditions represent four separate states, sometimes referred to as four quadrant control. The positive and negative voltage correspond to desired clockwise or counterclockwise rotation in the motor. The current corresponds to desired clockwise and counter clockwise torque. Two of these states correspond to normal motoring modes wherein both the voltage and the current have the same sense, either positive or negative. In the motoring modes, the motor is driven from the power source. The other two states correspond to regenerative modes, wherein the voltage and the current have an opposite sense, one being positive while the other is negative. In the regenerative mode, the internal generated motor current is utilized to produce desired torque.
In four quadrant reversing drives the transition from the forward conducting bridge to the reverse non-conducting bridge is normally handled to extinguishing the current in one bridge prior to allowing conduction in the other. Typically, control is to bring the offgoing bridge to full retard (firing angle of 180 degrees) and enable the oncoming bridge at full retard. This method ensures the extinction of current in the bridge before current is established in the other direction. The period of transition from extinction of one bridge to initial conduction of the other bridge is the basis for a deadband exhibited between forward and reverse current of flow. The deadband limits the outer speed regulator control loop band width. In other words, in the deadband region, control of the motor is severely limited. The minimum deadband is that which just allows the conducted current to become discontinuous prior to reversal when the next SCR fires in the oncoming bridge.
The essential condition for the reversal is that conducted current be discontinuous prior to the bridge reversal and that the average current level be sufficiently low as to form nearly linear response around the reversal. A problem of past digital and analog controllers is measuring inaccurately the discontinuous current for timing the bridge reversal. Another problem that occurs is measuring inaccurately current during the region of reversal. Any current signal during reversal represents inaccuracies in the current feedback circuitry which requires compensation. Prior digital and analog controllers had problems determining the oncoming firing angle such that it sufficiently advances to produce light conduction, eliminating the deadband that would have occurred if the angle had to advance from retard limit.
An inaccurate reading in the current during reversal is a considerable problem, because the magnitude of the current may be small and its wareshape flat. In addition, the ability to alter and optimize firing sequence in reversal is very desirable in order to provide smooth, continuous system operation.
Therefore, it is an object of the present invention that the average level of conducted current prior to reversal and subsequent to reversal exhibit a linear voltage characteristic across the reversal.
It is another object to extend the linear behavior of the current control and thus enable higher bandwidth of the speed control loop.
It is a further object of this invention to correctly select the initial firing angle on a reversal source to minimize the deadband.
It is a further object of this invention to base the choice of bridge on information that exhibits more accurate control over the actual bridge selection, even in the presence of offset error in the current feedback signal itself.